I use methylparaben, but it doesn't seem to do much to prevent mold. I use it more out of superstition than because I believe it makes some huge difference. I just happen to have a big bag of it from a friend so I add it, but I believe adding vinegar, yeast and a bunch of adult flies does more. Once the larvae start swimming around my cultures don't have mold issues. The consistency of the media seems a very important factor. Maybe adding larvae would help too if you have regular issues.